1. Field of the Invention
The present invention relates to an inkjet recording apparatus.
2. Description of the Related Art
As an inkjet recording apparatus that has a conveyance mechanism for conveying a recording medium such as a sheet of paper, the following inkjet recording apparatus is known. Examples of the conveyance mechanism include the one that has a conveying belt with a plurality of through-holes and having a sheet mounted on a front surface thereof, a conveying plate that is oppositely disposed along a lower surface of the conveying belt and provided with a plurality of air circulation holes on nearly the entire conveying plate, and negative pressure generating means disposed below the conveying plate and for suctioning the sheet through the air circulation holes of the conveying plate and the through-holes of the conveying belt. In an inkjet, a nozzle surface provided with a plurality of nozzles is disposed opposite to the front surface of the conveying belt.
In such a conveyance mechanism, not to mention it is important to stably suction air from all of the plurality of air circulation holes provided on the conveying plate in order to stably convey the sheet by means of the conveying belt. Particularly, the flow of the air in the vicinity of the inkjet head is important because it has a great impact on image formation.
In a conventional inkjet recording apparatus, there is known a conveyance mechanism that has air flow control means for performing control to reduce the amount of air at a predetermined region that extends from immediately below an inkjet head to a downstream side in a sheet conveying direction. This conveyance mechanism closes air circulation holes in a section corresponding to the predetermined region on a conveying platen, reduces the density of the air circulation holes formed in the section corresponding to the predetermined region, and reduces the diameter of each of the air circulation holes in the section corresponding to the predetermined region. Consequently, the amount of air decreases in the predetermined region, which, in turn, supposedly can prevent contamination of the sheet that is caused by ink mist generated at a front edge of the sheet.
Incidentally, in this conventional conveyance mechanism, the plurality of air circulation holes provided on the conveying platen are in a circular form. Therefore, many of the air circulation holes need to be arrayed over the entire conveying platen so that the sheet can be suctioned by substantially the entire conveying platen. With such many air circulation holes, the relative position between the sheet and the conveying platen changes during conveyance of the sheet, and consequently the suction power to suction the sheet fluctuates easily. In other words, if the degree in which the conveying platen and the sheet overlap in a thickness direction changes due to the change of the relative position, the rate of the air circulation holes closed (covered) by the sheet changes. A large negative pressure is obtained when the rate of the air circulation holes closed by the sheet increases, hence the suction power increases. When, on the other hand, the rate of the air circulation holes closed by the sheet is small, such as when the sheet is positioned on an upstream side or downstream side of the inkjet head, the suction power to suction the sheet is not sufficient. Specifically, in the latter situation, there are a number of unclosed air circulation holes from which a large amount of air is suctioned, and as a result a sufficient negative pressure for suctioning the recording medium cannot be obtained.
In addition, when the suction power to suction the sheet is not sufficient in a region facing the nozzle surface of the inkjet head, the front edge or rear edge of the sheet floats away from the conveying belt and abuts on the inkjet head.
On the other hand, large suction power of the air in the region facing the nozzle surface of the inkjet head promotes ink to be dried and become viscous at a plurality of nozzles provided on the nozzle surface of the inkjet head.